Re: High Mem Options

[Gerrit Huizenga <gh@us.ibm.com>]

On Wed, 05 Mar 2003 04:26:54 PST, William Lee Irwin III wrote:
> On Wed, Mar 05, 2003 at 06:28:36AM -0500, Reed, Timothy A wrote:
> > 	Yet another quick question...is there any down side to using the
> > 64GB option over the 4GB option if the machine only has 2GB of RAM onboard??
> > I would think this would be a performance issue?  Does the kernel only use
> > the translation table if it has to access any memory location over 4GB?
> 
> Yes, the additional level of pagetables slows things down quite a bit.

Bill, do you hvae measures for this?  I seem to remember PTX's impact
of PAE36 as being about 3-5% depending on workload.  Janet did one test
sometime back with DB2 that showed a net of no difference on TPC-H (PAE
slows things down, less memory pressure speeds things up) but Badari
just repeated with 2.5.62 or 2.5.63 and saw a larger degradation.

I'm wondering if some hardware is not getting correctly configured at
boot with with respect to MTRR's, perhaps...  I really wouldn't expect
a 10% impact from PAE and I don't have any consistent Linux measurements
to validate or invalidate that much impact.

gerrit